how to find out which christmas light is bad

I would suggest using a continuity tester to check for any short circuits or broken wires in the string of lights. If there's a problem with one bulb, it can cause the entire string to malfunction. By isolating the issue, you can quickly identify and replace the faulty bulb without having to go through the entire string. It's important to always check the manufacturer's instructions for safety precautions before working with electricity.